Known as the methods of producing fluoropolymers with a high ion-exchange capacity by copolymerizing a fluorine-containing ethylenic monomer, such as CF2═CF2 and a monomer containing such a functional group as a halosulfonyl group are the bulk polymerization and solution polymerization techniques. In particular, the solution polymerization technique is in frequent and general use.
Japanese Kokai Publication Hei-06-234816 discloses, as an example of the solution polymerization technique, a method of polymerization which uses a hydrochlorofluorocarbon as the polymerization solvent. However, when this method of polymerization is carried out, the viscosity in the reaction system increases and, as a result, it becomes necessary to stop the reaction before satisfactory yields are obtained after the start of the reaction. Conversely, if attempts are made to produce high yields, there arises the problem that the composition distribution becomes wide, resulting in deterioration in moldability.
Japanese Kokai Publication Hei-06-211933 discloses a method of polymerization which comprises using perfluorocyclobutane as the polymerization solvent. However, it has no description about the polymerization of a monomer containing such a functional group as a halosulfonyl group.
In Japanese Kokai Publication Hei-06-322034, there is disclosed a method which comprises subjecting a halosulfonyl group-containing monomer to polymerization in such a non-telogen solvent as perfluoroheptane, perfluorodimethylcyclobutane or a like perfluoroalkane or perfluorocycloalkane. This method, however, seems to be such that all the monomers to be subjected to polymerization are charged into a reaction vessel at the time of starting the polymerization. Thus, it has a problem, namely the composition distribution tends to become wide when the content of the halosulfonyl group-containing monomer is low.